In a large bowl, mix pork, salt, baking soda, pepper, sugar, soy sauce, oyster sauce, and sesame oil until just combined.
Add carrot, onion, garlic, and cabbage. Mix until evenly combined.
Add shrimp, shiitake mushrooms, cornstarch, and egg. Mix by hand until the filling comes together.
Place a wrapper over your hand and add about 1 tablespoon filling in the center. Press it down, gently squeeze the sides, then flatten the bottom so it stands upright.
Arrange siomai in a greased or lined steamer basket, leaving space between each piece.
Steam over medium heat for about 15 minutes, or until the filling is fully cooked.
Serve hot with your favorite dipping sauce.